Cells obtain energy in the form of ATP through the enzymatic degradation of glucose. This process begins with the glycolytic pathway in the cytosol when glucose is converted to pyruvate. Subsequently pyruvate is transported to mitochondria and converted to acetyl-CoA. Acetyl-CoA, whether obtained through the breakdown of glucose, fatty acids or amino acids, enters the citric acid cycle and, through the action of seven different enzymes, is converted to carbon dioxide and water. The citric acid cycle produces ATP, NADH and FADH.sub.2, and intermediates needed in lipogenesis, gluconeogenesis, and in the biosynthesis of amino acids.
Citrate synthase, the first enzyme in the citric acid cycle, catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate and Coenzyme A (CoASH). This reaction, the rate-limiting step of the citric acid cycle in many cells, is regulated by concentrations of acetyl-CoA, oxaloacetate, citrate, succinyl-CoA, and NADH. Pig citrate synthase has been purified and its amino acid sequence, three-dimensional structure, and mechanism determined (cf. Wiegand, G. and Remington, S. J. (1986) Ann. Rev. Biophys. Biophys. Chem. 15: 97-117). Citrate synthase is encoded by the nuclear DNA, translated in the cytosol, and transported into mitochondria via a leader sequence. The leader is cleaved to release the mature citrate synthase which localizes to the mitochondrial inner membrane. A sequence of thirteen amino acids surrounding the active site histidine is conserved among eukaryotic and prokaryotic citrate synthases (Karpusas, M. et al. (1990) Biochemistry 29:2213-2219). The active enzyme is a dimer of identical subunits found in nearly all living cells.
Citrate synthase activity can be measured in human tissue, and lowered citrate synthase activity is associated with cancer. Human glioblastoma multiforme (glioma), the most frequent malignant primary brain tumor, has increased glycolytic metabolism (Oudard, S. et al. (1997) Anticancer Research 17:1903-1912). When human glioma cells are transplanted into rat brain, citrate synthase activity is lower in the tumors than in normal rat brain. When the tumor cells are examined by electron microscopy, mitochondria are fewer in number than in normal rat brain and have altered morphology. The research suggests that the lowered number of mitochondria in glioma cells explains the reduced citrate synthase and increased glycolytic activities of these cells.
Lowered citrate synthase activity is associated with non-insulin-dependent diabetes mellitus (NIDDM). Patients with NIDDM, many of whom are obese, have lower citrate synthase activity in muscle biopsies than those of either lean or obese control subjects (Simoneau, J.-A. and Kelley, D. E. (1997) J. Appl. Physiol. 83: 166-171). Citrate synthase activity is also altered in patients with chronic renal failure. Chronic renal failure (CRF) is caused by a number of diseases, including chronic pyelonephritis, nephroangiosclerosis, chronic glomerulonephritis, polycystic kidney disease, and Alport's syndrome (Pastoris, O. et al. (1997) Scand. J. Urol. Nephrol. 31: 281-287). CRF patients often exhibit muscle weakness and fatigue, and citrate synthase activity in CRF patient muscle biopsies has been reported as lower than control (Conjard, A. et al. (1995) J. Am. Soc. Nephrol. 6: 68-74).
Lowered citrate synthase activity is associated with migraine headache. Patients who suffer from migraine, with and without aura, have lower citrate synthase activity in their platelet mitochondria than do controls (Sangiorgi, S. et al. (1994) Cephalalgia 14: 21-23). The authors suggest that this reduction in citrate synthase activity in migraine patients indicates a general impairment in energy metabolism and mitochondrial function.
Humans exhibit declining muscle mass and performance as they age. These traits are associated with a decrease in the activities of mitochondrial enzymes, including citrate synthase (Rooyackers, O. E. et al. (1996) Proc. Natl. Acad. Sci. USA 93:15364-15369). Citrate synthase activity was decreased in muscle homogenates and isolated mitochondria and suggests a decline in both muscle oxidative capacity and mitochondrial function. A reduction in physical activity also lowers citrate synthase activity. When endurance exercise-trained subjects stopped their physical training, their citrate synthase activity dropped (Coyle, E. F. et al. (1984) J. Appl. Physiol. 57: 1857-1864).
The discovery of a new human citrate synthase homolog and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of cancer, inflammatory disorders, and disorders of the sympathetic nervous system.